Certain types of products, including printed circuit boards for electrical apparatus, may be used in operating environments which subject such products to rigors well beyond those experienced by, e.g., a printed circuit board in a household television set. Military equipment, aircraft radar devices for example, are often required to be constructed of components which have been rigorously tested so that they are durable in such operating environments. One type of unusual operating environment involves temperature extremes. As an example of such an environment, a high performance military aircraft may fly from ground level (perhaps in the hot desert) to a very high, cold altitude in a very short time.
And manufacturers of such products and their customers have long been aware that product "stress screening" is an effective way to "prove" a product configuration and cull out products which may fail prematurely. Stress screening may include rapid sequential exposure to extremes of, for example, vibration, humidity or temperature. The invention relates to thermal stress screening during which products are repetitively exposed to temperature extremes, e.g., temperatures well below the freezing point and well above the boiling point of water.
It is common to thermally stress screen printed circuit boards by, for example, rapidly changing their temperature from about -20.degree. C. or even about -40.degree. C. (about equal to -6.degree. F. and -40.degree. F., respectively) up to about 125.degree. C. which is about equal to 260.degree. F. It is easy to understand why testing systems imposing such rapid temperature extremes are sometimes referred to as "thermo-shock" systems.
A known way to thermally stress screen circuit boards is to place such boards within a thermal stress chamber (often called an environmental test chamber) and alternately force very hot air and very cold air through the chamber for the time required to bring the board temperature to the required high or low level. Of course, the air used for a particular hot or cold cycle is simply drawn from the room ambient and heated or chilled, as needed. Examples of environmental test devices are shown in U.S. Pat. Nos. 3,656,058 (Leathers); 4,683,4224 (Cutright et al.) and 4,695,707 (Young). A leading manufacturer of environmental test chambers (as well as other stress screening products) is Thermotron Industries, Inc. of Holland, Mich.
While environmental test chambers using heated or chilled forced air have been highly satisfactory, using air as the thermal transfer medium requires a significant amount of time to repetitively change the temperature of the circuit board from one extreme of temperature to the other. Total process time on the order of a few hours for a particular group of boards being screened is not unusual. For a chamber cavity capable of holding a given number of circuit boards, it is apparent that there is some maximum number of boards that can be screened per unit time. Therefore, the product "throughput" for that test chamber can be calculated.
Efforts to shorten the total process time for a given number of circuit boards have included using liquid as the thermal transfer medium and contacting printed circuit boards directly with such liquid. With this arrangement, overall process times can be reduced by a ratio of perhaps 5:1 or even 10:1. Such efforts in this field have included using two liquid tanks (one each for hot and cold liquid) and a different type of liquid in each tank. Circuit boards are moved to one tank and then to the other and the particular type of liquid in a tank was selected in view of whether it will be used to heat or chill the boards.
Even with two tanks, the use of two different types of liquids ultimately results in "cross-contamination" and the liquids must be discarded or at least the contaminating liquid removed therefrom. Cross-contamination occurs since some liquid clings to the boards as they are being transferred to the other tank. More recently, the problem of cross-contamination has been resolved by using a single type of liquid in both tanks. But the problem of transferring a relatively large number of printed circuit boards (which may have appreciable total weight) between tanks still remains. And since printed circuit boards are usually electrically energized during thermal screening, the problem of connecting, disconnecting and re-connecting such boards (or using a rather long "pigtail" connection) also remains.
An improved thermal stress screening system and method which avoid transferring products between liquid tanks, which dramatically shortens overall process time and which conserves the thermal transfer liquid would be an important advance in the art.